The invention relates to high frequency induction heating in general, and more particularly to a current measuring transformer which is particularly adapted for control of an induction heating apparatus in response to coil current directly measured.
Control of the induction heating apparatus is essential for an efficient operation and for adapting an existing equipment and power supply to a wide range of workpieces of different shape, geometry, and material.
A customary approach with induction heating apparatus has been to control the voltage or the power of the coil circuit from the electrical power source. These methods have not been satisfactory because the final temperature for the workpiece treated is never obtained with sufficient precision and manual adjustment has been required in general.
Where the final temperature is critical, the prior art has made use of closed loop feedback control by direct comparison of the actual temperature with the desired temperature as a reference. In such case, an error signal is generated which causes a change in the power supply.
Instead of controlling the power supply in regard to temperature, magnetic forces have also been used as the control parameter, but this requires a strict and precise control of the current passing through the induction coil for any quality standard by heat treatment to be achieved.
An object of the present invention is to provide coil current control in an induction heating apparatus.
The invention rests on the observation that neither the voltage nor the power supplied to the tuned tank circuit has a direct relationship to the coil current.
Thus, for voltage control the coil current I.sub.C is given by the equation: ##EQU1## where Vo=coil voltage;
R=coil resistance; PA1 f=driving frequency; PA1 f.sub.o =resonant frequency of coil and tuning capacitors; PA1 L=coil inductance; PA1 C=tuning capacitor. PA1 I.sub.o =current fed to the tank circuit; PA1 .phi.=phase angle between current I.sub.o and voltage Vo.
For power control the coil current I.sub.C is given by the equation: ##EQU2## where in addition to the parameters of equation (1): Po=power applied to the tank circuit under Vo and I.sub.o ;
It appears that in both instances the coil current I.sub.C is dependent on the driving frequency from the power supply as well as upon the impedance of the coil. Since all the aforementioned parameters are susceptible of varying during the heating process, precise control cannot be achieved with either of these methods.
Direct coil current measurement is a serious problem with high frequency induction heating. Some processes incorporating high frequency induction equipment require precise control of coil current to properly control the end product. Such control demands the use of a coil current sensor that provides an accurate, representative current signal which can be conditioned and used for feedback information in the control system. Coil currents are generally 2 to 120 times the power supply current and most often are many thousands of amperes for processes requiring even modest powers (100 KW and up).
High frequency current measurements become more difficult with increasing frequency and amplitude of the current waveform. Although current shunts, magnetic pick-up devices, etc. are suitable, in principle, for the sensing element, current transformers provide reliable, accurate and economical alternatives. Properly designed and installed, the current transformer provides an isolated signal independent of frequency (within its design range). Conventional high current, high frequency current transformers using enameled wire wound on a 0.004 inch, 50% Ni-50% FE gain oriented tapewound cores suffice to levels of approximately 2500 amperes at 3 KHZ. However, at higher currents and/or frequencies the conventional approach is not effective.